linux-kernel - RE: [Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1

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Message-id: <003e01cc058f$94beb490$be3c1db0$%szyprowski@samsung.com>
Date:	Thu, 28 Apr 2011 12:32:32 +0200
From:	Marek Szyprowski <m.szyprowski@...sung.com>
To:	'Russell King - ARM Linux' <linux@....linux.org.uk>,
	'Benjamin Herrenschmidt' <benh@...nel.crashing.org>
Cc:	linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
	'Arnd Bergmann' <arnd@...db.de>,
	linux-arm-kernel@...ts.infradead.org
Subject: RE: [Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1

Hello,

On Thursday, April 28, 2011 11:38 AM Russell King - ARM Linux wrote:

> > > > 2. Implement dma_alloc_noncoherent on ARM. Marek pointed out
> > > >    that this is needed, and it currently is not implemented, with
> > > >    an outdated comment explaining why it used to not be possible
> > > >    to do it.
> > >
> > > dma_alloc_noncoherent is an entirely pointless API afaics.
> >
> > I was about to ask what the point is ... (what is the expected
> > semantic ? Memory that is reachable but not necessarily cache
> > coherent ?)
> 
> As far as I can see, dma_alloc_noncoherent() should just be a wrapper
> around the normal page allocation function.  I don't see it ever needing
> to do anything special - and the advantage of just being the normal
> page allocation function is that its properties are well known and
> architecture independent.

If there is IOMMU chip that supports pages larger than 4KiB then
dma_alloc_noncoherent() might try to allocate such larger pages what will
result in faster access to the buffer (lower iommu tlb miss ratio).
For large buffers even 64KiB 'pages' gives a significant performance
improvement.

Best regards
-- 
Marek Szyprowski
Samsung Poland R&D Center

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